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- 1. MSI Shift Registers• 74LS194 4-Bit Bidirectional Universal Shift Register• may be used in the following data register transfers – serial-serial, – shift left, – shift right, – serial-parallel, – parallel-serial, – and parallel-parallel 1
- 2. MSI Shift Registers• 74LS194 4-Bit Bidirectional Universal Shift Register 2
- 3. MSI Shift Registers• 74LS194 control inputs S1 and S0 3
- 4. MSI Shift Registers• 74LS194 4-Bit Bidirectional Universal Shift Register 01 11 10 00 01 11 10 00 01 11 10 00 01 11 10 00 4
- 5. MSI Shift Registers• 74LS194 4-Bit Bidirectional Universal Shift Register 5
- 6. “Universal”shift register 74x194• Shift left• Shift right• Load• Hold 6
- 7. MSI Shift Registers• One stage of the 74x194 7
- 8. MSI Shift Register 74195
- 9. MSI Shift Register 74195
- 10. MSI Shift Register 74195or D0–D3
- 11. MSI Shift Register 74195
- 12. MSI Shift Register 74195• 74195 logic diagram /P0 /P1 /P2 /P3
- 13. Ring Counter• A ring counter is a loop of flip-flops interconnected in such a manner that only one of the devices may be in a specified state at one time• If the specified state is HIGH, then only one device may be HIGH at one time.• As the clock, or input, signal is received, the specified state will shift to the next device at a rate of 1 shift per clock, or input, pulse. 14
- 14. MSI Shift Registers• 74LS194 control inputs S1 and S0 15
- 15. Shift-Register Counters• Ring counter• For Shift rightS0Vcc, S1 for Load and Reset, AVcc, BCD Gnd,QD RIN, LIN is not connected 16
- 16. Ring counter (Self correcting)• 4 bit, 4 state with a single circulating 1
- 17. State diagram for a self correcting ring counter 0001 0000 0010 1000 1001 0100 1100 1010 0110 1110 0101 1101 0011 1011 0111 1111
- 18. Ring counter (Self correcting)• 4 bit, 4 state with a single circulating 0
- 19. Johnson Counter (“Twisted ring” counter) 20
- 20. Timing diagram for a 4-bit Johnson counter 21
- 21. States of an 4-bit Johnson counterState Q3 Q2 Q1 Q0 DecodingName S1 0 0 0 0 Q3’•Q0’ S2 0 0 0 1 Q1’•Q0 S3 0 0 1 1 Q2’•Q0 S4 0 1 1 1 Q3’•Q2 S5 1 1 1 1 Q3•Q0 S6 1 1 1 0 Q1•Q0’ S7 1 1 0 0 Q2•Q1’ S8 1 0 0 0 Q3•Q2’* Can be decoded with 2-input gates
- 22. Self correcting Johnson Counter • n-bit counter • 2n - 2n unused states • 0x…x0 → 00…01 • 2 input NOR gate performs correction 23
- 23. Linear Feedback Shift Register Counter• n-bit shift register counters have far less than the maximum number of 2n normal states• n states for ring counter, 2n states for Johnson counter• An n-bit LFSR counter can have 2n – 1 states also called as maximum length sequence generator• Design is based on the theory of finite fields• Developed by French mathematician Evariste Galois• Serial input is connected to the sum modulo 2 of a certain set of output bits• These feedback connections determine the state sequence of the counter• By convention, outputs are always numbered and shifted in the direction shown in figure on next slide
- 24. • There exists at least one equation which makes the counter go through all the 2n - 1 states before repeating• It can never cycle through all 2n states• Regardless of the connection pattern 0…0 → 0…0 25
- 25. Feedbackequationsfor LFSRcounters 26
- 26. 3-bit LFSR counter Sequence X2 X1 X0 1 0 0 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 X2 X1 0 0 1 X0 1 0 0X3
- 27. Modified LFSR Counter• An LFSR can be modified to have 2n states including the all 0’s state• In an n-bit LFSR counter, an extra EXOR gate and an n – 1 input NOR gate connected to all the shift register outputs except X0 accomplishes the task• The states are not visited in binary order• Usually used where this characteristic is an advantage - Generating test inputs for logic circuits - Encoding and decoding circuits for certain error- detecting and error-correcting codes including CRC codes - Scrambling and descrambling data patterns in data communications - Pseudo random binary sequence generator
- 28. Modified 3-bit LFSR counter to include all 0’s Sequence X2 X1 X0 1 0 0 0 1 0 1 0 1 1 1 0 1 1 1 0 1 1 0 0 1 0 0 0 X2 1 0 0 X1 X0 X3

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